Typically, an x-ray beam generating device, referred to as an x-ray tube, comprises opposed electrodes enclosed within a cylindrical vacuum vessel. The vacuum vessel is typically fabricated from a glass tube or a cylinder made of metal, such as stainless steel, copper or a copper alloy. One of the electrodes comprises the cathode assembly which is positioned at some distance from the target track of a rotating, disc-shaped anode assembly. The impact zone of the anode is generally fabricated from a refractory metal with a high atomic number, such as tungsten or tungsten alloy. A typical voltage difference of 80 kV to 140 kV is applied across the cathode and anode assemblies. Thermal electrons are emitted by the hot cathode filament and accelerated across the potential difference impacting the target zone of the anode at high velocity. A small fraction of the kinetic energy of the electrons is converted to high energy electromagnetic radiation, x-rays, with the balance being converted to heat. The x-rays radiate from the focal spot in all directions. An x-ray transmissive window is fabricated into the vacuum vessel to allow the x-ray beam to exit at the desired location.
After exiting the vacuum vessel, the x-rays are directed to penetrate an object, such as human anatomical parts for medical examination and diagnostic procedures. The x-rays transmitted through the object are intercepted by a detector and an image is formed of the internal anatomy. Further, industrial x-ray tubes may be used, for example, to inspect metal parts for cracks or for inspecting the contents of luggage at airports.
The production of x-rays in a medical diagnostic x-ray tube is by its nature a very inefficient process. Typically less than one percent of the input power is converted to x-rays with the remainder being converted to heat in the anode. Consequently, the components in x-ray generating devices operate at elevated temperatures. For example, the focal spot on the anode can run as high as about 2700.degree. C., while the bulk of the anode ranges up to about 1700.degree. C. The excess heat from the anode must be transferred through the vacuum vessel and removed by a cooling fluid. Due to its close proximity to the focal spot, the x-ray window is subject to very high heat loads resulting from thermal radiation and back-scattered electrons from the target. These high thermal loads on the vacuum vessel x-ray transmissive window necessitate careful design to insure that the window remains intact over the life of the x-ray tube, especially in regards to vacuum integrity. Resulting large cyclic thermal stresses can cause vacuum leaks in the window joints resulting in premature failure of the x-ray tube.
The vacuum vessel is typically enclosed in a casing filled with circulating dielectric oil. The casing supports and protects the x-ray tube. Often the casing is lined with lead to provide stray radiation shielding. The oil often performs two duties, one is to cool the vacuum vessel by circulating over the vessel and drawing away the heat, and the second is to provide high voltage insulation between the anode and cathode connections. Alternatively, some prior art devices have attempted to cool the x-ray tube with circulating air. The casing, typically made from aluminum, operates at a much lower temperature than the vacuum vessel, since the casing is not directly exposed to the high temperature anode and back-scattered electrons.
X-ray tubes with glass vacuum vessels typically do not include separate x-ray transmissive windows since the x-ray attenuation of glass in the medical diagnostic energy range, approximately 80 kV to 150 kV, is relatively low. Glass tubes use the vacuum vessel wall as the window. However, for x-ray tubes having metal vacuum vessels (typically made from stainless steel or a copper alloy), an x-ray transmissive window must be attached to an opening cut into the metal vessel because the x-ray attenuation (absorption) of the metal wall is very large.
A number of characteristics are considered desirable when choosing an x-ray transmissive window for an x-ray tube vacuum vessel. First, the x-ray attenuation coefficient of the window material must be small over the x-ray energy range of interest so that the maximum x-ray flux is transmitted. Second, the window must be able to withstand the high temperature operating environment of the x-ray tube. Third, the window material must be able to be joined to the vacuum vessel forming a reliable hermetic seal under atmospheric pressure and high thermal stresses.
The window should be relatively thin, on the order of 1 mm, to maximize x-ray throughput. As such, the window is generally fabricated from low atomic number materials, which inherently have low x-ray attenuation. This generally precludes any window materials of atomic number greater than that of titanium (atomic no. 22). Therefore, neither copper (atomic no. 29) nor stainless steel windows can be effectively used. The two most common methods of joining materials in x-ray tubes is high temperature brazing and welding. Welding is most applicable to joining similar metals. The differing materials typically used for the x-ray transmissive window and the vacuum vessel, thus generally do not lend themselves to welding. Reliable vacuum system brazing is generally performed with braze filler metals with liquidus points higher than 650.degree. C. Therefore, the window material must be able to withstand the high brazing temperature.
In prior art x-ray generating devices, beryllium has been the material of choice for transmissive x-ray windows in metal x-ray tube vacuum vessels for a number of reasons. Beryllium has an extremely low x-ray attenuation coefficient that allows transmission of virtually all levels of x-rays. The attenuation coefficient of a material is related to the material's atomic number. Beryllium has an atomic number of 4, and as such is one of the most transmissive materials available. Also, beryllium possesses a high melting point, 1277.degree. C., low vapor pressure, and good thermal conductivity, thus making beryllium an excellent material for the vacuum window. Additionally, because of its high melting point, beryllium can be brazed to the metal wall of the vacuum vessel, thereby providing a hermetic seal.
However, beryllium does have some serious drawbacks, especially with regard to ease of manufacture, safety, and cost. The machining and processing of beryllium require special precautions due to the toxicity of beryllium dust. At elevated temperature, an oxide of beryllium forms on its surface which can become dispersed in the environment if not properly handled. Beryllium is also a somewhat brittle material, so it is difficult to fabricate into complicated shapes. Further, because beryllium has such a low attenuation coefficient, it transmits low energy x-rays as well as diagnostic x-rays. In many instances, the lower energy x-rays simply add to the dose given to the patient, necessitating further attenuation by additional filters. Typically when a beryllium window is used, another x-ray filter must be added downstream of the x-ray tube to block out the lower energy x-rays. Thus, beryllium has a number of significant drawbacks as an x-ray transmissive window.
Another transmissive window material used in the prior art is titanium. The attenuation coefficient of titanium is much larger than that for beryllium, consequently, a titanium window must be very thin to provide comparable x-ray transmission. The relative thinness of a titanium window creates a structural problem that limits the size of the window because of the force due to atmospheric pressure on the window. Additionally, the thermal properties of titanium are quite poor in comparison to beryllium. The poor thermal properties of titanium result in very high window temperatures and thermal stresses. Consequently, titanium also has a number of significant drawbacks as an x-ray transmissive window.
Aluminum transmissive windows have been utilized in x-ray applications, such as for windows in the x-ray tube casing or as windows for image intensifier units, but generally not in the high temperature and high stress environment of an x-ray tube vacuum vessel. U.S. Pat. No. 4,045,699, U.S. Pat. No. 4,153,854 and U.S. Pat. No. 4,763,042, for example, disclose aluminum x-ray transmissive windows utilized in radiation image intensifiers. Radiation image intensifiers are vacuum vessels comprising electronic components that convert radiation into electrons to provide a illuminated image of the object subjected to the radiation. Image intensifiers typically operate near room temperature, as their delicate electronic components can be damaged or malfunction at high temperatures. Further, image intensifiers are not subject to very large mechanical and thermal operating stresses. Therefore, the environment of a transmissive window in an image intensifier is significantly different from the environment of an x-ray tube vacuum vessel transmissive window.
A two-layered x-ray transmissive window is disclosed in U.S. Pat. No. 4,045,699 for use in an image intensifier. One layer is formed of a light weight metal comprising the x-ray transmissive portion, the second layer is formed of a heavy weight metal comprising the weldable transition to the metal frame of the image intensifier. The specifically declared materials for the light weight material are aluminum and titanium and the heavy weight material being copper or iron. This layered material is formed in a commercial process by rolling the two materials under high pressure. This type of joining process is suitable for vacuum vessels operated at low temperature, such as an image intensifier. However, this type of joint would not form a reliable, long term seal in the high temperature, high stress environment of an x-ray tube vacuum vessel. The aluminum-copper and aluminum-iron joint is subject to the formation of an intermetallic layer, which makes the joint brittle and reduces the integrity of the vacuum seal, especially when subjected to the aggressive thermal cycling of an x-ray tube vacuum vessel. Additionally, as pointed out in U.S. Pat. No. 4,153,854, assigned to the same entity as U.S. Pat. No. 4,045,699, the two-layered sheet formed by high pressure rolling is disadvantageous because it lacks uniform quality and especially because it does not possess a uniform gas-permeable adherence between the two-layers. Additionally, the two-layered window was intended for use in the relatively passive, low temperature environment of a radiation image intensifier, rather than the high stress, high temperature environment of an x-ray tube vacuum vessel. As such, the reliability of the vacuum seal between the two-layered window as disclosed in U.S. Pat. No. 4,045,699 is not sufficient for use in an x-ray tube vacuum vessel.
Thus, there is a need for an x-ray transmissive window in a metal x-ray tube vacuum vessel with low x-ray attenuation that solves the above problems.